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Tectonic significance of paired gravity anomalies in the southern and central Appalachians

By
M. D. Thomas
M. D. Thomas
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Published:
January 01, 1983

Extensive, positive-negative paired gravity anomalies occur along a number of Precambrian boundaries separating terranes with contrasting structural characteristics and radiometrically determined ages. Collective evidence from the boundaries supports an origin by plate collision following convergent plate movements operating in the style of modern day plate tectonics. The paleosubduction direction can be determined from the relative ages of the sutured terranes, the younger one being identified as a “reactivated” terrane formed in response to subduction beneath it. Because the positive-negative pattern of gravity anomalies is invariable with respect to the relative ages of the sutured terranes, the paleosubduction direction can be established with confidence on the basis of the gravity pattern alone; the latter also identifies the suture position.

In the southern and central Appalachians, such utility of the basement is prevented by a widely-preserved cover of supracrustal rocks. Geological indicators of paleosubduction direction and suturing are present in these rocks, but because of the determined or possible allochthonous nature of some of the critical terranes, locating deeper, more fundamental sections of suture zones is problematical. However, the presence of paired gravity anomalies, strikingly similar to Precambrian examples, affords a novel method for identifying the location of a deep suture and the associated paleosubduction direction. Studies of these anomalies lead to the conclusion that a plate carrying a proto-American block subducted southeastward, leading to collision with an “accreted” block, in agreement with the general consensus that such a collision occurred in the Ordovician.

Along the COCORP seismic profile, gravity modelling indicates that the suture dips southeastward beneath the Inner Piedmont and transects a master dêcollement outlined by the seismic studies. The presence of a suture in this locality suggests that any thin-skinned tectonic model for this part of the Appalachians should be restricted to the area northwest of the suture.

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Contents

GSA Memoirs

Contributions to the Tectonics and Geophysics of Mountain Chains

Robert D. Hatcher, Jr.
Robert D. Hatcher, Jr.
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Harold Williams
Harold Williams
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Isidore Zietz
Isidore Zietz
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Geological Society of America
Volume
158
ISBN print:
9780813711584
Publication date:
January 01, 1983

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